TESTS: Fix E501 pep8 issues on test_netgroup.py
    
    E501: line too long (longer than 79 characters)
    
    The issue was inserted in commit 0f8add07b8, which has been pushed just
    before the pep8 patches got merged.
    
    The whole file was changed in order to adapt to the changes proposed to
    this patch.
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>